1. Field of the Invention
The present invention relates to a method of manufacturing a solar cell having a silicon layer comprised of a silicon thin film, and more particularly to a method of manufacturing a solar cell of crystalline silicon thin film.
2. Description of the Prior Art
In a solar cell comprised of single-crystal or polycrystal silicon thin film, the silicon layer that generates electricity is thin, having a thickness ranging from several microns to several tens of microns, which is not enough to support itself like a cast substrate can that is in the order of 300 microns thick. It therefore requires some sort of support substrate.
Therefore, in fabricating such a solar cell, it is necessary to directly deposit a silicon film on the support substrate by a method such as chemical vapor deposition (CVD), or to bond a silicon film that has been fabricated on another substrate or a silicon thin-film solar cell that has been fabricated on another substrate onto the required support substrate.
The silicon thin film can be bonded onto the support substrate by various methods such as a method comprising forming a layer of single-crystal on a layer of porous silicon formed on a silicon substrate, and after the solar cell has been fabricated, adhering the solar cell to the support substrate, then separating and removing the silicon substrate, or a method comprising forming a silicon thin film on an oxide layer formed on the silicon substrate, adhering it to the required support substrate, then separating and removing the silicon substrate.
To improve the energy conversion efficiency, a high-concentration impurity layer is formed on the back surface of the solar cell to produce a field effect that prevents recombination of minority carriers. This is referred to as a back surface field (BSF) structure.
However, there are problems with the prior art methods of adhering a silicon film to a support substrate. For example, a high-concentration diffusion layer has to be formed prior to the bonding step, but limitations on the control of the diffusivity concentration and diffusion depth make it difficult to form the high-concentration diffusion layer. Moreover, when the silicon film is going to be directly deposited onto the support substrate, first a high-concentration diffusion layer is deposited, and then the predetermined silicon film is formed, so the procedure is a complicated one. In the case of the prior art manufacturing methods described above, it is difficult to precisely control the formation of the high-concentration impurity diffusion layer required for the BSF structure.
The object of the present invention is to provide a method of manufacturing a solar cell that enables the high-concentration impurity diffusion layer to be formed with good precision, simplifies the manufacturing steps and improves the photoelectric conversion efficiency.
To attain the above object, the present invention provides a method of manufacturing a solar cell comprising interposing an intermediate layer containing p-type or n-type impurity between a silicon thin film and a support substrate, and subjecting all or part of the structure thus formed to heat treatment at 500xc2x0 C. to 1500xc2x0 C. to diffuse the intermediate layer impurity into the silicon thin film to form a high-concentration impurity layer in the silicon thin film.
The silicon of the silicon thin film may be single-crystal silicon or polycrystal silicon.
The impurity in the intermediate layer may be a group III element such as boron or aluminum or a compound thereof such as boron oxide, or a group V element such as phosphorus, antimony or a compound thereof such as phosphorus oxide.
The silicon thin film may be formed directly on the intermediate layer, or a silicon thin film formed on a thin silicon substrate or other substrate may be adhered to the intermediate layer.
Since the method of manufacturing a solar cell according to the present invention employs a step of forming a high-concentration impurity layer in the silicon thin film by heat-treating an intermediate layer that contains the impurity, it is not necessary to diffuse a high-concentration impurity layer before adhering the silicon thin film, so the manufacturing procedure is simplified.
Moreover, the concentration and diffusion depth of the impurity diffusion layer can be controlled with good precision by adjusting the impurity concentration of the intermediate layer, the temperature at which heat treatment is effected and the length of the heat treatment.
Also, since the high-concentration impurity diffusion layer can thus be formed with good precision, it is possible to effectively control the recombination of minority carriers, making it possible to provide solar cells having improved photoelectric conversion efficiency. Furthermore, without any additional processes it is also possible to form a back side field layer or back side electrode layer, which also helps to increase the efficiency of the solar cells.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.